"A warning from U.S. scientists that Oklahoma may be hit by a major earthquake has caused a run on insurance policies for tremors in the heartland state, adding to the woes of residents already in the firing line of devastating tornadoes.

Quakes have typically been infrequent in Oklahoma, yet not unheard of. But in the past year, minor tremors have hit the state hundreds of times, raising worries the big one may be just around the corner.

"The rate of earthquakes in Oklahoma has increased by about 50 percent since October 2013, significantly increasing the chance for a damaging quake in central Oklahoma," the U.S. Geological Survey (USGS) said earlier this week.

Geologists also say that fracking - a controversial technique for extracting natural gas and oil from underground rock deposits - could be one of the causes."